Processes are disclosed in the art for converting ingots of metal into a molten state and ultimately converting the molten metal into metal powder. Various methods have been employed. In the basic “gas atomization process” molten metal is passed through a nozzle into an ejection chamber wherein it is mixed with a continuous incoming stream of gas under pressure. The gas serves to atomize the stream of molten metal, which upon cooling converts into a metal powder. A variation of gas atomization (“ultrasonic gas atomization”) employs incoming gas injected through a convergent-divergent nozzle at ultrasonic speed into a mixing chamber wherein it is mixed with a spray of molten metal. The impact of the supersonic gas jet on the wall of the chamber produces a shock wave which helps to disintegrate the molten metal into small droplets. The metal droplets solidify outside the chamber into a metal powder upon cooling in a controlled atmosphere. Other methods employ centrifugation to disintegrate an incoming stream of the molten metal into fine liquid droplets which convert to metal powder upon cooling.
In the “centrifugal shot casting process” a batch of the metal is placed into a water cooled crucible. A stationary electrode in proximity to the crucible is activated to an electric arc between the electrode and crucible. This causes sufficient heating to melt the metal. As the crucible spins, centrifugal force causes the molten metal to move up the wall of the crucible. As the molten metal moves to the edge of the spinning crucible it breaks up and is ejected by centrifugal force in the form of droplets, which solidify into metal particles under a controlled atmosphere of argon or helium. This process was originally used to atomize refractory powders for use in nuclear fuels, but it has been used to produce a wide range of metal powders including iron, nickel, cobalt, and titanium.
In the “internally cooled spinning disk atomization” process a stream of molten metal is injected under pressure into a hollow cup shaped cavity within the disk core. Simultaneously a wall of cool liquid quenchant is directed against the inside wall of the cup causing the molten metal to disintegrate into small liquid droplets. The metal powder is formed as the liquid droplets condense in a controlled atmosphere.
In other centrifugal processes “centrifugal atomization” for producing a metal powder from molten metal there are no internal quenching of the molten metal within the cup core. The molten metal may be injected directly into the cup-shaped core of a spinning disk without injecting a quenchant into the cup core. As the disk rotates at high speed the molten metal forms a film on the surface of the cup core. As the film reaches the periphery of the edge of the spinning disk it begins to break up into small droplets. The small liquid metal droplets solidify into a metal powder within a chamber held under a controlled atmosphere. The conventional disk for such atomization process has a cup shaped cavity without any baffles therein. Conventional disk spinning rates for such atomization process for production of zinc powder, is typically between about 500 and 8000 rpm (revolutions per minute), for example between about 1000 and 8000 rpm. With such conventional process and disk spinning rates the typical D50 median of the zinc particles produced in a typical batch production is between about 200 and 350 micron. (The D50 median and mean average particle size of a batch of zinc particles produced by the atomization process are generally of about the same value, so these terms can effectively be used interchangeably.)
It is desired to improve the mechanical design of the “spinning disk” to reduce the chance of slippage of the molten zinc on the surface of the spinning disk. Slippage results in loss of centrifugal force on a given mass of molten zinc and thus a reduction in kinetic energy as a given mass of molten zinc exits from the edge of the spinning disk and breaks up into liquid droplets. Such loss in centrifugal force can in turn result in larger droplets and consequently larger particle size zinc powder product than is desired.
It is desired to improve the centrifugal atomization process so that a zinc powder having a greater portion of smaller size zinc particles can be produced. Such zinc powder can improve performance in alkaline cells.